In vitro inhibitory effect of somatostatin on secretin action in exocrine pancreas of rats

Adenylyl cyclases in the digestive system

Adenylyl cyclases (ACs) are a group of widely distributed enzymes whose functions are very diverse. There are nine known transmembrane AC isoforms activated by Gαs. Each has its own pattern of expression in the digestive system and differential regulation of function by Ca(2+) and other intracellular signals. In addition to the transmembrane isoforms, one AC is soluble and exhibits distinct regulation. In this review, the basic structure, regulation and physiological roles of ACs in the digestive system are discussed.

Adenylyl cyclase 6 mediates the action of cyclic AMP-dependent secretagogues in mouse pancreatic exocrine cells via protein kinase A pathway activation

Both secretin and vasoactive intestinal polypeptide (VIP) receptors are responsible for the activation of adenylyl cyclases (ACs), which increase intracellular cyclic AMP (cAMP) levels in the exocrine pancreas. There are nine membrane-associated isoforms, each with its own pattern of expression and regulation. In this study we sought to establish which AC isoforms play a regulatory role in pancreatic exocrine cells. Using RT-PCR, AC3, AC4, AC6, AC7 and AC9 were found to be expressed in the pancreas. AC3, AC4, AC6 and AC9 were expressed in both pancreatic acini and ducts, whereas AC7 was expressed only in pancreatic ducts. Based on known regulation by intracellular signals, selective inhibitors and stimulators were used to suggest which isoforms play an important role in the induction of cAMP formation. AC6 appeared to be an important isoform because protein kinase A (PKA), PKC and calcium all inhibited VIP-induced cAMP formation, whereas calcineurin or calmodulin did not modify the response to VIP. Mice with genetically deleted AC6 were studied and showed reduced cAMP formation and PKA activation in both isolated pancreatic acini and duct fragments. The absence of AC6 reduced cAMP-dependent secretagogue-stimulated amylase secretion, and abolished fluid secretion in both in vivo and isolated duct fragments. In conclusion, several AC isoforms are expressed in pancreatic acini and ducts. AC6 mediates a significant part of pancreatic amylase and fluid secretion in response to secretin, VIP and forskolin through cAMP/PKA pathway activation.

The role of somatostatin in 67 consecutive pancreatectomies: a randomized clinical trial

BACKGROUND

Somatostatin has been found to be effective in the prevention of postoperative complications in pancreatic surgery. It can inhibit the pancreatic secretions that, quite often, are responsible for complications during the postoperative period.

METHODS

We randomized 67 patients in 2 groups. In the study group (n = 35), somatostatin was administered 30 minutes prior to surgery as well as intraoperatively and postoperatively. No medication was given to the control group (n = 32). Biopsies were taken and processed for electron microscopy and ultrastructural morphometric analysis.

RESULTS

Administration of somatostatin reduced the exocrine granule number, and the patients suffered from fewer postoperative complications.

CONCLUSIONS

Somatostatin reduces granule number and size of pancreatic cells, which can partially explain the prophylactic effect of the drug on early complications of pancreatic surgery, and which is confirmed by the clinical findings.

The islet-acinar axis of the pancreas: more than just insulin

Although the role of the islets in the regulation of acinar cell function seemed a mystery to investigators who observed their dispersion among pancreatic acini, over time an appreciation for this intricate and unique structural arrangement has developed. The last three decades have witnessed a steadily growing understanding of the interrelationship of the endocrine and the exocrine pancreas. The islet innervation and vascular anatomy have been more fully characterized and provide an appropriate background for our current understanding. The interrelationship between the endocrine and exocrine pancreas is mediated by islet-derived hormones such as insulin and somatostatin, other humoral factors including pancreastatin and ghrelin, and also neurotransmitters (nitric oxide, peptide YY, substance P, and galanin) released by the nerves innervating the pancreas. Although considerable progress has been achieved, further work is required to fully delineate the complex interplay of the numerous mechanisms involved. This review aims to provide a comprehensive update of the current literature available, bringing together data gleaned from studies addressing the actions of individual hormones, humoral factors, and neurotransmitters on the regulation of amylase secretion from the acinar cell. This comprehensive view of the islet-acinar axis of the pancreas while acknowledging the dominant role played by insulin and somatostatin on exocrine secretion sheds light on the influence of the various neuropeptides on amylase secretion.

Microinjection of exogenous somatostatin in the dorsal vagal complex inhibits pancreatic secretion via somatostatin receptor-2 in rats

Previous studies have suggested that somatostatin inhibits pancreatic secretion at a central vagal site, and the dorsal vagal complex (DVC) is involved in central feedback inhibition of the exocrine pancreas. The aim of this study was to investigate the effect of exogenous somatostatin in the DVC on pancreatic secretion and the somatostatin receptor subtype(s) responsible for the effect. The effects of somatostatin microinjected into the DVC on pancreatic secretion stimulated by cholecystokinin octapeptide (CCK-8) or 2-deoxy-d-glucose (2-DG) were examined in anesthetized rats. To investigate the somatostatin inhibitory action site, a somatostatin receptor antagonist [SRA; cyclo(7-aminoheptanoyl-Phe-d-Trp-Lys-Thr)] was microinjected into the DVC before intravenous infusion of somatostatin and CCK-8/2-DG. The effects of injection of a somatostatin receptor-2 agonist (seglitide) and combined injection of somatostatin and a somatostatin receptor-2 antagonist (CYN 154806) in the DVC on the pancreatic secretion were also investigated. Somatostatin injected into the DVC significantly inhibited pancreatic secretion evoked by CCK-8 or 2-DG in a dose-dependent manner. SRA injected into the DVC completely reversed the inhibitory effect of intravenous administration of somatostatin. Seglitide injected into the DVC also inhibited CCK-8/2-DG-induced pancreatic protein secretion. However, combined injection of somatostatin and CYN 154806 did not affect the CCK-8/2-DG-induced pancreatic secretion. Somatostatin in the DVC inhibits pancreatic secretion via somatostatin receptor-2, and the DVC is the action site of somatostatin for its inhibitory effect.

Efficacy of lanreotide 30 mg on prevention of pain relapse after oral refeeding in patients with necrotizing acute pancreatitis. A phase II prospective multicentre study

BACKGROUND

Pain relapse after oral refeeding occurs in 21% of the patients with acute pancreatitis, and in 35% of those with CT Balthazar's score > or =D [Gut 1997;40:262]. Somatostatin analogues may decrease the pain relapse rate by inhibiting exocrine pancreatic secretion. AIMS, PATIENTS AND METHODS: To assess the frequency of pain relapse in patients with acute necrotizing pancreatitis after treatment with one intramuscular injection of lanreotide 30 mg on the day before refeeding. The refeeding procedure was standardized and progressive.

RESULTS

23 patients were included in 4 centres. Acute pancreatitis was alcoholic (n = 11), biliary (n = 7), other (n = 5). Twelve patients had > or =3 Ranson's criteria. Balthazar's score (1985) was D or E in 7 and 16 patients, respectively. Median duration of pain and of interruption of oral feeding were 11 (3-23) and 16 (5-34) days, respectively. Median hospital stay was 22 (9-41) days. Only 1 patient (4.3 %) had pain occurring 3 days after refeeding.

CONCLUSION

Pain relapse occurred in 4.3% of patients pretreated with the somatostatin analogue lanreotide, and this figure is lower than the expected 35% rate which was previously reported without preventive treatment. This suggests that one intramuscular injection of lanreotide 30 mg on the day before refeeding could decrease pain relapse in patients with acute necrotizing pancreatitis, but has to be confirmed in a phase III study.

Regulation of carbachol-induced c-fos mRNA expression in AR42J cells by somatostatin receptor subtypes 1, 2, and 3

INTRODUCTION

Somatostatin is an inhibitory peptide that exerts its effects tissue-specifically by activating one or more of five receptors (SSTR 1-5). Although several studies have examined which SSTR subtypes control gastrointestinal function, effects of somatostatin on pancreatic gene expression are not well defined.

AIM

To examine the effects of somatostatin and newly synthesized selective SSTR agonists on the cholinergically stimulated expression of the immediate early response gene

METHODOLOGY AND RESULTS

In pancreatic acinar AR42J cells, polymerase chain reaction analysis revealed that mRNAs for SSTR 1, 2, and 3 were expressed. SSTR 4 and 5 were not detected. When AR42J cells were exposed to the cholinergic agonist carbachol in the presence of somatostatin or selective SSTR agonists, significant and dose-dependent reductions in agonist-induced levels of mRNA were noted. Pretreatment with agonists specific for SSTR 4 or 5 had no inhibitory effects. The inhibitory actions of somatostatin were pertussis toxin-sensitive. In addition, since somatostatin did not affect intracellular calcium homeostasis, the inhibitory actions of somatostatin are independent of calcium signaling.

CONCLUSION

The current studies demonstrate that somatostatin inhibits carbachol-induced increases in expression by interacting with somatostatin receptor subtypes 1, 2, and 3. In addition, because somatostatin did not affect intracellular calcium homeostasis, it can be concluded that SSTR actions are independent of carbachol-stimulated calcium signaling.

In vivo octreotide administration acutely reduces exocrine granule size in the human pancreas

BACKGROUND

Octreotide has been found to be effective in the prevention of postoperative complications of pancreatic surgery, but the benefit of preoperative octreotide administration has not been assessed.

AIMS

To evaluate the ability of octreotide in reducing the amount of digestive enzymes in the pancreas before surgery, a morphometric ultrastructural study of the gland was undertaken in patients undergoing demolitive pancreatic surgery.

METHODS

Twenty-three inpatients received saline (n = 8) or octreotide (100 micrograms s.c.) before surgery either once (n = 5), or three (n = 5) or six (n = 5) times at 8-hour intervals. At surgery, biopsies of the pancreas were taken and processed for electron microscopy. Several parameters were assessed in exocrine cells by means of ultrastructural morphometry.

RESULTS

A single administration of octreotide significantly reduced the exocrine granule number and the mean and total granular surface sectional area, and the ratio between granule area and cytoplasmic area. Repeated octreotide administrations were associated with partial (3 administrations) and complete (6 administrations) recovery of all parameters to control values.

CONCLUSION

Preoperative administration of octreotide, the synthetic analogue of somatostatin, acutely reduces exocrine granule number and size in the pancreatic cell. This finding can partially explain the prophylactic effect of the drug on early complications of pancreatic surgery. Such an effect is not maintained over multiple administrations of the somatostatin analogue. Possible explanations for this latter finding are discussed.

Endogenous somatostatin inhibits interaction of insulin and cholecystokinin on exocrine secretion of isolated, perfused rat pancreas

INTRODUCTION

Although somatostatin inhibits pancreatic exocrine secretion, the inhibitory mechanism of endogenous somatostatin is not clearly understood.

AIM

To investigate the effect of endogenous somatostatin on the interaction between endogenous insulin and exogenous cholecystokinin (CCK) in exocrine secretion of the totally isolated, perfused rat pancreas.

METHODOLOGY

Endogenous releases of somatostatin and insulin were induced by 18 mM glucose. Streptozotocin (75 mg/kg) or cysteamine (300 mg/kg) was injected into rats 24 hours before the experiment to deplete insulin or somatostatin in the pancreas.

RESULTS

Glucose (18 mM) enhanced CCK (10 pM)-stimulated secretions of fluid and amylase in the normal pancreas, which was further elevated by a somatostatin antagonist. Exogenous insulin (100 nM) also enhanced CCK-stimulated secretions in the streptozotocin-treated pancreas, which was also markedly increased by the somatostatin antagonist. The glucose (18 mM)-enhanced CCK-stimulated secretions were much higher in the cysteamine-treated pancreas than in the normal pancreas, which was dose-dependently reduced by exogenous somatostatin (30, 100 pM). However, endogenous or exogenous somatostatin did not modify the pancreatic responses to CCK alone.

CONCLUSION

Endogenous somatostatin inhibits the interaction of endogenous insulin and CCK on pancreatic exocrine secretion in the rat rather than reducing the action of CCK alone or endogenous release of insulin.

Effect of somatostatin on cholecystokinin-induced amylase release in rat pancreatic acini

The effect of somatostatin on cholecystokinin-induced amylase release was investigated in isolated rat pancreatic acini. Acini were isolated by enzymatic digestion and incubated in a HEPES buffered Ringer's solution with testing reagents for 30 minutes at 37 degrees C. The activity of released amylase, cAMP, and inositol phosphate formation were measured. Intracellular calcium concentration ([Ca2+]i) was also checked. Somatostatin 14 and octreotide, a somatostatin analog, inhibited CCK-stimulated amylase release in a concentration-dependent manner. The inhibitory effect of octreotide on CCK-induced amylase release was not shown when the acini were treated with 8-Br-cAMP, irrespective of the presence of IBMX. Forskolin potentiated CCK-induced amylase release and this effect was blocked by octreotide treatment; although CCK-8 (3 x 10(-11) M) failed to stimulate cAMP formation, octreotide significantly inhibited basal cAMP formation in the acini. The increase of [Ca2+]i in response to CCK was inhibited by octreotide. However, CCK-induced inositol phosphate formation was not changed by 10(-9) M octreotide. Octreotide had no effect on CCK-stimulated tyrosine phosphorylation, and tyrosine phosphatase inhibitors (NaF and Na2WO4) did not influence the effect of octreotide on CCK-induced amylase release. From these results, we conclude that octreotide inhibits CCK-induced amylase release by inhibiting basal cAMP formation and decreasing the [Ca2+]i stimulated by CCK.

Effect of nitric oxide on the somatostatinergic system in the rat exocrine pancreas

Nitric oxide (NO) and somatostatin (SS) are two important mediators of the exocrine and endocrine pancreas, exerting opposite effects on this organ. There is strong evidence suggesting an interaction between pancreatic NO and SS. The aim of this study was to determine whether L-arginine (L-Arg), the substrate for NO synthase (NOS), and Nomega-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, regulate pancreatic somatostatin-like immunoreactivity (SSLI) content and the SS mechanism of action in pancreatic acinar cell membranes. L-Arg (150 mg/kg, intraperitoneally (i.p.)), L-NAME (50 mg/kg, i.p.) or L-NAME plus L-Arg were injected twice daily at 8 h intervals for 8 days. L-Arg decreased pancreatic SSLI content as well as the number of SS receptors in pancreatic acinar cell membranes whereas L-NAME increased both parameters. The stable SS analogue SMS 201-995 induced a significantly lower inhibition of forskolin-stimulated adenylyl cyclase activity in pancreatic acinar cell membranes from L-Arg-treated rats whereas an increased inhibition was observed in pancreatic acinar membranes from L-NAME-treated rats. These results indicate that the NO system may contribute to the regulation of the pancreatic somatostatinergic system.

Inhibition of stimulated amylase secretion by adrenomedullin in rat pancreatic acini

Adrenomedullin is a novel hypotensive peptide originally isolated from human pheochromocytoma and recently localized to PP cells of the pancreatic islets of Langerhans. Based on the pancreatic islet-acinar axis model, we investigated the effect of adrenomedullin on regulated exocytosis of exocrine pancreas. Using rat [125I]-adrenomedullin, specific binding sites were localized to rat pancreatic acini. We next examined the effect of adrenomedullin on 100 pM cholecystokinin (CCK)-stimulated amylase release from pancreatic acini. Adrenomedullin inhibited amylase secretion in a dose-dependent manner by approximately 50% at maximum, and the IC50 was 1.1 pM. However, adrenomedullin did not affect rat [125I]CCK binding to isolated acini or reduce the intracellular free Ca2+ concentration increased by CCK. Adrenomedullin also inhibited amylase secretion induced by 1 microM calcium ionophore A23187, suggesting that adrenomedullin inhibits stimulated amylase secretion by functioning at a step(s) distal to the ligand-receptor binding system and intracellular calcium mobilizing mechanism. In streptolysin-O permeabilized acini, 10 nM adrenomedullin shifted the calcium dose-response curve to the right, indicating that adrenomedullin inhibits calcium-induced amylase secretion by reducing calcium sensitivity of the pancreatic exocytotic machinery. In addition, pretreatment of pancreatic acini with pertussis toxin abolished the inhibitory effect of adrenomedullin on CCK-stimulated amylase secretion. These results indicate that adrenomedullin inhibits stimulated amylase secretion by reducing the calcium sensitivity of the exocytotic machinery of the pancreatic acini. A pertussis toxin-sensitive GTP-binding protein(s) is also involved in this mechanism.

Heterogeneity of the proliferative capacity of rat cholangiocytes after bile duct ligation

We previously introduced the concept that intrahepatic bile duct epithelial cells, or cholangiocytes, are functionally heterogeneous. This concept is based on the observation that secretin receptor (SR) gene expression and secretin-induced cAMP synthesis are present in cholangiocytes derived from large (> 15 microns in diameter) but not small (< 15 microns in diameter) bile ducts. In work reported here, we tested the hypothesis that cholangiocytes are heterogeneous with regard to proliferative capacity. We assessed cholangiocyte proliferation in vivo by measurement of [3H]thymidine incorporation and in vitro by both [3H]thymidine incorporation and H3 histone gene expression in small (fraction 1) and large (fraction 2) cholangiocytes isolated from rats after bile duct ligation (BDL). In the two cholangiocyte subpopulations, we also studied basal somatostatin receptor (SSTR2) gene expression as well as the effects of somatostatin on 1) SR gene expression and secretin-induced cAMP synthesis and 2) [3H]thymidine incorporation and H3 histone gene expression. In normal rat liver, cholangiocytes, unlike hepatocytes, were mitotically dormant; after BDL, incorporation of [3H]thymidine markedly increased in cholangiocytes but not hepatocytes. When subpopulations of cholangiocytes were isolated after BDL, DNA synthesis assessed by both techniques was limited to large cholangiocytes, as was SSTR2 steady-state gene expression. In vitro, somatostatin inhibited SR gene expression and secretin-induced cAMP synthesis only in large cholangiocytes. Moreover, compared with no hormone, somatostatin inhibited DNA synthesis solely in large cholangiocytes. These results support the concept of the heterogeneity of cholangiocytes along the biliary tree, extend this concept to cholangiocyte proliferative activity, and imply that the proliferative compartment of cholangiocytes after BDL is located principally in the cholangiocytes lining large (> 15 microns) bile ducts.

Significant cholinergic role in secretin-stimulated exocrine secretion in isolated rat pancreas

Effects of intrapancreatic cholinergic activation by electrical field stimulation (EFS) on secretin-stimulated pancreatic exocrine secretion were investigated in the totally isolated perfused rat pancreas. EFS at 15 V, 2 ms, and 8 Hz for 45 min markedly increased spontaneous pancreatic secretion. This increase was completely inhibited by tetrodotoxin (1 microM) but not by hexamethonium (100 microM). Atropine (2 microM) significantly reduced the EFS-evoked volume flow and amylase output by 52% and 80%, respectively. EFS further increased the secretin (12 pM)-stimulated pancreatic secretion of fluid and amylase. The increases of the two parameters were significantly suppressed by atropine by 28% and 72%, respectively. Interestingly, EFS significantly increased concentrations of somatostatin-like immunoreactivity in portal venous effluents. When pertussis toxin (200 ng/ml) or rabbit antisomatostatin serum (0.1 ml/10 ml; titer of 1:50,000) was intra-arterially administered, EFS further increased the secretin-stimulated pancreatic secretion. In conclusion, the activation of intrapancreatic cholinergic neurons potentiated the secretin action on pancreatic exocrine secretion in the rat. This potentiating effect was significantly reduced by local somatostatin released during EFS that activated intrapancreatic cholinergic tone.

Effects of lanreotide, a somatostatin analogue, on postprandial gastric functions and biliopancreatic secretions in humans

AIMS

Lanreotide is a novel synthetic somatostatin analogue. A long-acting formulation of lanreotide has been shown to be effective for the treatment of gastroentero-pancreatic hormone-producing tumours but effects on postprandial digestive and absorptive functions remain obscure. The aim of the present study was to evaluate the effects of intravenous lanreotide on gastric and biliopancreatic secretions in man as well as the absorption of nutrients and the duodeno-caecal transit time after ingestion of an homogenized meal (500 kcal, 55% carbohydrates, 15% proteins, 30% lipids).

METHODS

Eight healthy male volunteers were studied on two occasions within a 2 weeks interval, using a perfusion method. They received in single-blind and random order continuous i.v. infusion of either placebo or lanreotide (100 micrograms h-t after a bolus of 100 micrograms 15 min before the beginning of the study).

RESULTS

Lanreotide significantly decreased gastric acid secretion (90%) for the initial 3 h period. Gastric emptying was not significantly modified by lanreotide infusion. Compared with placebo, lanreotide almost completely abolished both bile salts and lipase responses to the meal. It largely increased the duodeno-caecal transit time and decreased significantly the duodenal absorption of carbohydrates and triglycerides.

CONCLUSIONS

Since lanreotide has powerful effects on gastrointestinal functions, it could be useful in the prevention or in the treatment of pancreatic and bowel fistulas as well as short bowel syndrome.

Chronic pancreatitis: diagnosis and treatment

Three-dimensional magnetic resonance cholangiopancreatography is currently the most exciting new imaging technique for chronic pancreatitis. Endoscopy-assisted duodenal intubation during the secretin-cholecystokinin test reduces intubation time in difficult cases. The NBT-para-amino benzoic acid test has been refined to enhance its discriminant power. The cholesteryl-[C13]octanoate breath test and the faecal elastase test are newer highly sensitive and specific tubeless tests. Pain in chronic pancreatitis continues to be a vexing therapeutic issue. Enzyme treatment continues despite criticism. Neurotensin is the new suspected mediator of the feedback mechanism, which is downregulated by enzyme therapy. Steroid ganglion block is an exciting therapeutic tool for pain relief. Endoscopic pancreatic sphincterotomy, Dormia basketing and pancreatic stenting in conjunction with extracorporeal shock wave lithotripsy should be performed early in chronic pancreatitis to prevent parenchymal atrophy with ensuing exocrine and endocrine pancreatic dysfunction. The modified Puestow's procedure preserves endocrine and exocrine pancreatic functions besides relieving pain. Closed loop insulin infusion allows superior management of pancreatic diabetes following near total pancreatectomy. The standardised incidence rate of pancreatic cancer is 16.5 in patients with alcoholic chronic pancreatitis and 100 for tropical chronic pancreatitis. Aggressive treatment protocols combining neo-adjuvant chemoradiation and intra-operative radiation with surgery are being used to improve the prognosis in this dismal complication of chronic pancreatitis.

Inhibition by somatostatin of amylase secretion induced by calcium and cyclic AMP in rat pancreatic acini

It has recently been shown that somatostatin inhibits amylase secretion from isolated pancreatic acini by reducing cyclic AMP (cAMP) production [Matsushita, Okabayashi, Hasegawa, Koide, Kido, Okutani, Sugimoto and Kasuga (1993) Gastroenterology 104, 1146-1152]. To date, however, little is known as to the other mechanism(s) by which somatostatin inhibits amylase secretion in exocrine pancreas. To investigate the action of somatostatin independent of cAMP generation, we examined the effect of somatostatin in isolated rat pancreatic acini stimulated by 1 microM calcium ionophore A23187 and 1 mM 8-bromo-cyclic AMP (8Br-cAMP). Somatostatin inhibited amylase secretion evoked by a combination of A23187 and 8Br-cAMP in a dose-dependent manner. The maximum inhibition was obtained by 10(-7) M somatostatin, and at this concentration somatostatin inhibited the effect of A23187 and 8Br-cAMP by approximately 30%. In electrically permeabilized acini, an elevation of free calcium concentration resulted in an increase in amylase secretion and cAMP enhanced the secretion evoked by calcium. cAMP shifted the dose-response curve for calcium-induced secretion leftwards and elevated the peak value of secretion. Somatostatin inhibited the effect of cAMP on calcium-induced amylase secretion by shifting the dose-response curve to the right. To determine the involvement of a G-protein(s), we examined the effect of somatostatin in acini pretreated with pertussis toxin. Pretreatment of acini with pertussis toxin completely blocked somatostatin-inhibition of amylase-secretion evoked by A23187 and 8Br-cAMP. These results indicate that somatostatin decreases amylase secretion induced by cAMP and calcium by reducing the calcium sensitivity of exocytosis. A pertussis toxin-sensitive G-protein is also involved in this step.